The present invention relates to a tap drive unit coupled to the machine-head of a machine tool such as a machining center for cutting screw-threads inside cavities of workpieces by power feeding a tap.
In known machine tools such as a machining center, a tap is coupled to a main spindle of the machine tool. The machine-head of the machine tool, supporting the main spindle, is moved toward a workpiece in accordance with rotation of the main spindle.
When the machine-head of the machine tool is moved in synchronization with the rotation of the main spindle, the main spindle of the machine tool is required to be provided with a rotary encoder for detecting the rotation of the main spindle to perform an arithmetic operation, thereby controlling the axial movement of the machine-head of the machine tool. However, it is difficult, on the point of accuracy, to move the machine-head of the machine tool in accordance with the pitch of the tap.
The prior art provides an improvement wherein a chuck is supported on the main spindle of the machine tool through a coil spring. The chuck is adapted to be axially displaced slightly. In the case of a workpiece having a small thickness, however, the accuracy with which the screw-thread is formed in the workpiece is reduced owing to the spring forces acting on the chuck. Furthermore, it is impossible for the tap to perform the threading operation where the machine tool is not provided with the rotary encoder.